Matrix 4 multiplication test

[chapulina]

Closes #190

Summary

Added a test explicitly checking the result of Matrix 4 multiplication. I believe this addresses the concerns in #190. I ran the tests with both ign-math4 and ign-math6 and the test passes for both.

Test it

Compile and run ./bin/UNIT_Matrix4_TEST

Checklist

• Signed all commits for DCO
• Added tests
• Added example world and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review
  another open pull request
  to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #191 (b176445) into ign-math4 (00088ff) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math4     #191   +/-   ##
==========================================
  Coverage      71.59%   71.59%           
==========================================
  Files             67       67           
  Lines           8805     8805           
==========================================
  Hits            6304     6304           
  Misses          2501     2501           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 00088ff...b176445. Read the comment docs.